微信小程序获取到小程序二维码之后转base64，图片不显示

let senceRes = await uniCloud.httpclient.request('https://api.weixin.qq.com/cgi-bin/token', {  
    method: 'GET',  
    data: {  
        grant_type: 'client_credential',  
        appid: 'xxxxxxxxxx',  
        secret: 'xxxxxxxxxxx'  
    }  
})  
const access_token = JSON.parse(senceRes.data.toString()).access_token  
let imgRes = await uniCloud.httpclient.request('https://api.weixin.qq.com/wxa/getwxacodeunlimit?access_token=' +  access_token, {  
    method: 'POST',  
    data: {  
        scene: 'invite_code=' + inviteCode,  
        path: 'page/index/index',  
        width: 680,  
        auto_color: true,  
        is_hyaline: true  
    }   
})  
const base64 = imgRes.data.toString('base64')

返回了buffer图片，但是转base64之后，返回前端，前端加前缀之后不显示